In order to enable utilization as widespread as possible of a gas laser, it is desirable not only to be able to change the mirrors at the ends of the resonant cavity, but further to change the type of gas employed as an amplifying medium and contained within the discharge tube.
The laser model 2020 offered by the Company Spectra-Physics, Mountain View (USA), consists of a sealed discharge tube terminated by a pair of Brewster windows. This tube is mounted in a chassis which carries the mirrors. Such system, referred to as modular, gives ready free access to the mirrors since the latter are located outside the sealed tube, but it does not enable working with different gases and if such were to be the case, it would be necessary to change the entire tube, this being moreover foreseen in the notice published by the company in question. Here it should be also mentioned that the employment of Brewster windows is undesirable since these windows bring about a serious loss of power, such power being reduced to about half of that which one may attain if such windows are absent. The laser, as mentioned hereinabove, nevertheless enables changing the mirrors without touching the discharge tube. This possibility is desirable, initially for maintenance reasons, since it is necessary to be able to replace a mirror which is simply defective or the reflecting coating of which has become altered. Finally, one must be able to employ mirrors, the curvature of which is different in order to obtain different transverse electromagnetic modes (TEM). Finally, in changing the nature of the reflecting coating placed on the mirror, one may modify the spectral composition of the laser beam.
In contrast to the arrangement which has just been described above, the laser apparatus described in European patent No. 0.048.690 enables changing the gas contained in the discharge tube. To this effect are provided a first pumping valve and a second filling valve. However, here the mirrors are in direct communication with the discharge tube and it will be understood that it will not be possible to change them without bringing the entire interior of the discharge tube to atmospheric pressure. This presents the difficulty of having once again to evacuate the cavity following changing of the mirror and before introducing the gas, and takes considerable time which may extend for large dimension lasers over several days. On the other hand, if it were possible to isolate the mirrors of the discharge tube, one could during this operation maintain a very low pressure in the tube so as to enable rendering the laser once again operational after very little time lapse. It will be observed that changing the gas enables modification of the spectral composition of the laser beam; for instance, argon produces a blue and green tint and krypton a yellow and red tint.
It is the purpose of the present invention to overcome the difficulties presented by one or the other of the lasers previously described by placing into operation means permitting for the same laser apparatus a change of mirrors in the cavity and/or replacement of the gas by another gas without exposing the discharge tube to atmospheric pressure.